This invention relates to an eye testing and examining instrument such as an eye-fundus camera, an ophthalmoscope, etc. and more particularly to a method and a device for adjustment of a working distance between a human eye and an objective lens of such an instrument.
Spacing an eye examining instrument from the front of a human eye such as a cornea or an iris substantially affects the quality of the resulting image. Such spacing has heretofore been carried out by having the instrument moved by an operater who looks through a view finder in the optical axis direction of its objective lens until a flare or a ghost disappears from view.
However, when a flare is slight, the observer tends not to notice it and takes a photograph under incorrectly spaced conditions. Thus such flares often escape the observers' notice until after a photograph is finished. A photograph with such a flare is not usable for an accurate diagnosis.
Further, adjustment of such an instrument into a position in which there arises no flare or ghost arises not only takes time but also requires some skill and is not easy for observers.
On the other hand, there is known a device which illuminates an eye with an invisible light such as infrared rays or a very weak visible light; then detects a beam coming from the eye with a pick-up tube; and displays an electrical signal coming from the pick-up tube on a display surface of a cathode ray tube as an image of the eye. However, with such a device, a flare or ghost can be hardly detected, because a cathode-ray tube or an image intensifier used for such a purpose has a low resolving power and is inferior in sharpness or because the image is in a monochromatic light. Therefore, it has been often difficult to adjust a working distance with such a device.